This invention relates to bipolar junction transistors (BJTs), and more particularly to controlling parameters of a BJT fabricated on a substrate by applying a bias voltage to the substrate.
In bipolar junction transistors, the output current, or collector current, is exponentially dependent on the input voltage, or base-emitter voltage. This is different from MOSFET where the output current (Id) is more or less linearly dependent on input voltage, or gate voltage (Vg). This gives bipolar an advantage to be used in driver circuits where high current is needed to drive a load. However, BJTs have the limitation of needing relatively large input voltage to deliver sufficient current level. Unlike MOSFET where low threshold voltage can be achieved by tuning the work function of the gate material, BJTs made of silicon has a turn-on voltage around 0.9V-1V, which is dictated by the silicon band gap. Adding germanium to the base region can lower the turn on voltage due to the smaller bandgap of SiGe alloy as compared to silicon. However, this adds process complexity and cost. In addition, the amount of tuning is limited to the band gap shrinkage.